1. Field of the Invention
The invention relates to a method for producing a compound from a flowable plastic material and a solid fiber core wherein continuous fibers of reinforcing material which are wetted with plastic material are fed to the plastic material in the mixing zone of an extrusion chamber. Furthermore, the invention relates to a device for carrying out this method.
2. Description of the Related Art
U.S. Pat. No. 6,221,293 discloses a method for producing a compound from a plastic material and a solid fiber core in which a heated screw-type extruder is used, into which continuous fibers are drawn by drawing themselves in, are cut up there and mixed with the plastic material, so that subsequently a finished fiber compound can be delivered. To be able to keep the length of the mixing zone required for intensive mixing with the flowable plastic material in the extruder as short as possible, the continuous fibers are wetted in an impregnating die with the plastic material being used before they enter the extruder.
With regard to the impregnating die, reference is made to a commercially available device which is described in U.S. Pat. No. 4,883,625. This known device for wetting continuous fiber material is designed in the form of an extrusion die which has a continuous channel for the leading through of the fiber material. In the initial region of this channel, two flow channels for the feeding of flowable plastic material to the continuous fiber material end. The channel does not proceed in a straight line in the longitudinal direction, but instead has a number of kinks in the downward or upward direction, so that, when it is being led through, the fiber material is in each case deflected around corresponding edges. The intended effect of this is to ensure that the fiber material is wetted as uniformly and completely as possible. deflected around corresponding edges. The intended effect of this is to ensure that the fiber material is wetted as uniformly and completely as possible.
The object of the present invention is to develop a method of the generic type to the extent that uniform and complete wetting of the fiber material is ensured in as simple a way as possible. At the same time, it is intended that a compound which deliberately contains a particularly high proportion of very long fibers can be produced, to ensure a high breaking strength of the finished parts which can be produced from this compound. Furthermore, a device for carrying out this method is to be specified.
The invention is based on the idea that the production of a compound from a flowable thermoplastic material and a fixed fiber core of a reinforcing material takes place by extruding the materials being used from an extrusion chamber. The flowable plastic material may in principle also be thermosetting liquid polymers such as for instance polyester resins. However, thermoplastic which are plasticated in the extrusion chamber by supplying heat, for example polyethylene or polypropylene, are preferably used. Any desired materials may be used as the fiber material for reinforcing the flowable plastic material used in the sense of a matrix material. They are preferably glass fibers. The fiber reinforcing material is fed in the form of continuous fibers to the plastic material which is located in the extrusion chamber and already flowable and is mixed with the latter. Before the continuous fibers are introduced into the extrusion chamber, they are wetted with an amount of the intended flowable plastic material that is small in comparison with the amount of plastic material in the extruder. This prior wetting makes it possible to minimize the mixing work in the extruder to achieve a distribution of the fibers that it is as uniform as possible. As a result, the fibers fed in, which tend to break or tear under the action of the mixing tool used, can maintain a comparatively great length. The longer and more intensely the mixing work is carried out, the shorter the individual pieces of fiber become. In order that the products produced by further processing from the compound produced according to the invention achieve as high a breaking strength as possible, it is important however that the length of the fibers remains as long as possible. Without adequate wetting of the continuous fibers fed in, a much longer mixing operation would have to be carried out.
To achieve complete wetting of the continuous fibers with the flowable plastic material, the invention provides that the continuous fibers are led through a roller nip which is formed between a pair of wetting rollers driven in opposite rotational directions. In this arrangement, the surface of these wetting rollers has a coating of the plastic material of the matrix material in flowable form in a thickness sufficient for the constant filling of the roller nip with the plastic material. By leading the continuous fibers through the roller nip, the continuous fibers are inevitably brought into extremely intensive contact with the flowable plastic material and are consequently wetted completely with the plastic material on the surface in a very uncomplicated way.
In principle, it is possible to produce the flowable plastic material for coating the wetting rollers with a separate extruder and feed it to the surface of the wetting rollers. However, it is particularly advantageous to take the flowable plastic material required for this purpose from the extrusion chamber used for mixing with the continuous fibers as a partial stream at a point at which the plastic material is already flowable but has not yet been mixed with the fibers.
On account of their particularly good mixing properties, multi-screw extruders, preferably twin-screw extruders, are suitable for the production of the compound. In principle, however, single-screw extruders can also be used. The feeding in of the wetted continuous fibers expediently takes place in the region of the mixing zone of such an extruder. This mixing zone need have only a comparatively short length for adequate mixing with the matrix material.
To achieve particularly good wetting of the continuous fibers, it is recommendable to operate the wetting rollers in each case with circumferential roller speeds that are different from each other. In this way, a friction occurs in the roller nip, leading to fibrillation of the fiber material. This allows the individual fibers of the fiber strands fed in to be brought into particularly intensive contact with the flowable plastic material in the roller nip and be completely enveloped. The fibrillation successfully allows fibers that are sticking together, for instance due to xe2x80x9ccoatingxe2x80x9d, to be broken up and to achieve wetting of the xe2x80x9cindividual fibersxe2x80x9d. Different circumferential roller speeds can be achieved for example with the same rotational drive speed by choosing correspondingly different roller diameters. The circumferential roller speeds are realized, however, by corresponding setting and regulation of different rotational speeds of the wetting rollers.
To ensure orderly feeding of the continuous fibers to the wetting rollers, it is recommendable to lead the continuous fibers initially over a pair of drawing-in rollers, rolling on each other, and to pass them from these rollers into the roller nip of the wetting rollers. The drawing-in rollers, like the wetting rollers, each rotate oppositely in relation to the other, in order to achieve the material transport in the desired direction.
The device according to the invention for carrying out the method described above has as a major part a screw-type extruder, the extrusion chamber of which, surrounded by an extruder barrel, has in addition to the two openings for the entry of the plastic material of the matrix material and the exit opening for the extruded material a further lateral opening which serves for the feeding in of the continuous fiber material. The opening for the fiber material consequently lies between the entry opening and the exit opening for the plastic material, to be precise generally much closer to the exit opening. A further important part of the device is the wetting device for the fiber material. The outlet of this wetting device is in this case in transporting connection with the opening for the continuous fiber material in the extruder barrel. According to the invention, the wetting device has at least one pair of interacting wetting rollers, which are driven such that they rotate in opposite directions. In principle, it is also possible to arrange a number of pairs of such wetting rollers one behind the other or one next to the other. However, in general it is advisable to limit the wetting rollers to a single pair. The roller nip existing between the wetting rollers forms the outlet of the wetting device. In the region of the periphery of the wetting rollers, i.e. in the vicinity of their circumference, a feed for the flowable plastic material required for the wetting is arranged in such a way that the outer surface of the wetting rollers is constantly coated with the flowable plastic material during operation.
It is recommendable in the interests of a compact design to mount the wetting rollers in an enclosing housing which is firmly connected to the extruder barrel. This consequently means that this housing is arranged in the region of the lateral opening for the feeding in of the fiber material. The lateral opening is expediently provided in the region of the mixing zone of the screw-type extruder. In front of the housing in which the wetting rollers are mounted there is preferably also arranged a pair of drawing-in rollers, driven such that they rotate in opposite directions, by means of which the continuous fibers of the fiber material can be led in a controlled manner into the roller nip of the wetting rollers.
In principle, the screw-type extruder may have a single screw in the extrusion chamber. Better mixing is achieved, however, on multi-screw extruders. The use of twin-screw extruders is particularly preferred, the screws of the twin-screw extruder preferably being driven such that they rotate in opposite directions.
To create friction in the roller nip to assist complete wetting of the fiber material, in principle wetting rollers with different diameters may be used, so that they can be driven at the same rotational speed although different circumferential speeds at the outer roller surface are desired. It goes without saying that it is also possible with diameters of the wetting rollers that are the same as one another for desired different circumferential roller speeds to be achieved by a different gear transmission in the drive of the individual rollers. It is recommendable to design the rotary drives of the wetting rollers in such a way that their circumferential speed can be set and regulated independently of one another. This can be achieved in the simplest way by the rotary drives of the wetting rollers each being designed such that they are completely separate from one another.
With regard to the feed for the flowable plastic material to the wetting rollers, it is regarded as advantageous to design these feeds each as a branch channel which branches off upstream in the transporting direction of the screws of the screw-type extruder of the opening for the feeding of the fiber material from the extrusion chamber and ends openly in the direct proximity of the outer surface of the respectively assigned wetting roller, so that, as a consequence of the pressure in the flowable plastic material brought about by the screw or screws, a partial stream of this plastic material is applied through the branch channel to the outer surface of the wetting rollers.